Method for making thermoplastic films



Dec. 31, 1968 V D. J. RYAN 3,419,650

METHOD FOR MAKING THERMOPLASTIC FILMS Original Filed June 5, 1964 23IS'Q: I5 ,-23

United States Patent 3,419 650 METHOD FOR MAKING THERMOPLASTIC FILMSDaniel J. Ryan, Chester, Pa., assiguor to FMC Corporation, Philadelphia,Pa., a corporation of Delaware Original application. June 5, 1964, Ser.No. 372,909, new

Patent No. 3,355,768, dated Dec. 5, 1967. Divided and this applicationJune 8, 1967, Ser. No. 644,599

6 Claims. (Cl. 264-146) ABSTRACT OF THE DISCLOSURE A method for makingoriented films in which an extruded tubing of thermoplastic material,which is at a temperature at which the molecules thereof may be orientedby stretching, is engaged only along its internal surface by movablemeans which are advanced along paths and at such rates of speed as tocause the tubing to be stretched and thus oriented at least along one ofits longitudinal and transverse directions.

This application is a division of application Ser. No. 372,909, filed onJune 5, 1964, now US. Patent No. 3,- 355,768.

This invention relates to the manufacture of films and more particularlyto an improved method for making oriented films from tubing formed ofthermoplastic polymeric materials.

In a conventional method of making films from tubing, a freshly extrudedtubing is generally expanded by a bubble of fluid which is containedbetween the extrusion die and a pair of nip rolls, after which thetubing is slit into two or more films. A critical object to this andother similar procedures is that the contained bubble of fluid effectsstretching of the tubing immediately as it issues from the extrusiondevice so that the molten and thus more yieldable portion of tubing ismade thinner without providing for any appreciable degree of orientationtherein. In other words, the expansive effect of the bubble of fluid isdissipated in stretching the portions of the tubing which are in a moreyieldable condition but which are not capable of retaining orientation.Proposals for remedying the above-described disadvantage have beensatisfactory to some degree but, in general, lack the flexibility whichwould permit their use with diiferent materials and often requireextremely accurate and continuous control. Accordingly, a primary objectof this invention is the provision of an improved method for makingoriented films from tubing formed of thermoplastic polymeric materials.

Another object is the provision of an improved and simple method foreffecting a controlled uniaxial or biaxial stretching of an extrudedthin-walled tubing of thermoplastic polymeric material.

Still further :objects are apparent from the following description.

The objects are accomplished in accordance with the present invention byan improved method in which an extruded tubing of heated or moltenthermoplastic polymeric material is cooled to a temperature range withinwhich the thermoplastic material may be oriented by stretching, afterwhich the internal surface of the tubing is engaged along at least twocircumferentially spaced locations and then stretched at least along oneof its transverse and longitudinal directions. Transverse stretching ofthe tubing may be achieved by directing the engaged portions thereofalong diverging paths, while longitudinal stretching is best effected byadvancing the engaged tubing portions at a faster rate than the rate atwhich the tubing is extruded. During or after stretching of the tubing,the tubing is cooled to a temperature below the orientation temperaturerange and is then slit into two or more films.

The apparatus employed in the method of the present invention is one inwhich movable means such as one reach of each of a plurality of endlesscables is continuously engaged with the internal surface of a freshlyextruded tubing as it issues from an annular die after which suchopposing reaches are caused. to travel preferably along paths whichdiverge relative to the tubing axis. In this manner the tubing isstretched and oriented in a transverse direction concomitantly with itsadvancement. At locations spaced from the annular die the orientedtubing is slit into films at the locations at which it is engaged bysaid movable means and the paths of the endless cables are reversed. Theendless cables are led into the extruded tubing by a guide tube whichextends through the extrusion die and is adjustable relative thereto sothat the locations at which the cables first engage with the internalsurface of the tubing may be varied. Means are provided for positivelydriving the endless cables whereby the tubing may be advanced at such arate as to effect longitudinal orientation thereof alone orsubstantially concomitantly with its transverse stretching.

The method of the present invention is not limited for use with anyparticular thermoplastic polymeric materials or any specific set ofoperating conditions. Further, it will be apparent that the orientationimparted to the thermoplastic material may extend transversely or bothtransversely and longitudinally of the resulting films and that in thelatter instance may but need not be balanced along both axialdirections.

In the drawing, FIGURE 1 is a diagrammatic illustration of the apparatusemployed in the method of the present invention in use; and

FIGURE 2 is a transverse section taken substantially along the linesII-II of FIGURE 1..

With reference to the drawing, FIGURE 1 diagrammatically illustrates aportion of a conventional screw extruder 9 having an annular die 11 fromwhich a molten thermoplastic polymeric material may be continuouslyextruded as a seamless thin-walled tubing 13. A ring 15 is provided fordirecting cool air or other gas against the extruded tubing 13 to chillthe same to within a range of temperatures at which the particularthermoplastic may be oriented. At locations spaced from the die 11 thetubing 13 is slit by knives 17 into a plurality of films 19 which arecollected upon separate take-up cores 21 driven at substantially thesame rate of speed by suitable means, not shown.

The tubing 13 is stretched in a transverse direction concomitantly withits advancement by endless cables 23, with at least two, and preferablythree, of such cables being employed. The opposing reaches of theendless cables 23 are led up into and through the annular extrusion die11 by a guide tube 25 and are engaged with the internal surface of thetubing 13 as they travel along diverging paths toward and over pulleys27. The tubing 13 is slit as it approaches the pulleys 27, with both theslitting knives 17 and the pulleys 27 being mounted on carriages 29which are adjustably fixed to support plates 31. From the pulleys 27 thecables are laced about pulleys 33 which are all driven at substantiallythe same rate of speed by motor 35.

As illustrated on the drawing, the guide tube 25 is mounted for movementrelative to the extrusion die 11 and is provided with recesses 37 forreceiving a spring loaded retaining pin 39. The exit end of the tube 25is widened to permit a smooth and gradual outward movement of theendless cables and, if desired, may be fitted with rollers to minimizefriction.

In operation, molten thermoplastic polymeric material is delivered bythe extruder 9 and is: shaped into the form of the tubing 13 as itissues from the extrusion die 11. Chilled air from the ring 15 serves tocool the tubing 13 to such an extent that it is within the orientationtemperature range when its internal surface is continuously engaged bythe opposing reaches of the movable means, for example, endless cables23. The temperature of the cool air delivered by the ring 15 may bevaried to suit the particular polymeric material being extruded. It ispreferred, however, that the temperature of the quenching air remainsubstantially constant and that the distance of free travel of thetubing 13 be varied by adjusting the position of the guide tube 25relative to the die 11.

Once engaged with the internal surface of the tubing 13, the endlesscables carry and stretch the same in a transverse direction until it iscut by the knives 17 into films 19.

The degree of transverse stretching may, of course, be varied byadjusting the positions of the carriages 29 radially relative to theaxis of the tubing 13. The portions of the tubing with the movable meanstravel at substantially the same rate as that at which the tubing isextruded. Further, if desired, the motors 35 may drive the endlesscables 23 and the engaged portions of the tubing 13 at a rate of speedwhich exceeds the rate of extrusion so as to provide for longitudinalstretching of the tubing concomitantly with its transverse orientation.Regardless of the rate at which the endless cables are advanced, norelative movement exists between the cables and the portions of thetubing engaged therewith. Additionally, it will be apparent that theseparation between the endless cables 23 may be varied from thesubstantially equal spacing illustrated on the drawing, and that eitherthe cable spacing and/or the position of the knives 17 may be changed toprovide films 19 which are of different widths.

It is to be understood that changes and variations may be made withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

I claim:

1. A method of making oriented thermoplastic film including the steps ofextruding from an annular die a heated thermoplastic material in theform of a tubing, cooling the extruded tubing to within a temperaturerange at which molecular orientation of the thermoplastic material maybe efiected by stretching, while within said temperature range engagingonly the internal surface of the extruded tubing at least along twocircumferentially spaced locations with means adapted to move with saidtubing, orienting the molecules of the thermoplastic material bydirecting the movable means along paths and at such rates as to causethe tubing to be stretched at least in one direction, maintaining theinternal surfaces of the tubing continuously engaged with the movablemeans so that they travel together without relative movementtherebetween, slitting the oriented tubing into at least two films, andreleasing the tubing from the movable means concomitantly with theslitting thereof.

2. A method as defined in claim 1 wherein orientation of the moleculesof the thermoplastic material is effected by stretching the tubing in atransverse direction as the movable means are directed along paths whichdiverge relative to each other.

3. A method as defined in claim 2 including the step of cooling thetubing to a temperature below the orientation temperature range of thethermoplastic material prior to slitting of the same.

4. A method as defined in claim 2 wherein said tubing is slit at thelocations at which it is engaged by said movable means.

5. A method as defined in claim 2 wherein the portions of the tubingengaged with the movable means travel at substantially the same rate asthat at which the tubing is extruded.

6. A method as defined in claim 2 wherein the portions of the tubingengaged with the movable means travel at a faster rate than that atwhich the tubing is extruded to thereby stretch .and provide fororientation of the thermoplastic material in a longitudinal directionsubstantially concomitantly with its stretching and orientation.

References Cited UNITED STATES PATENTS 2,852,813 9/1958 Longstreth264-146 3,265,552 8/1966 Berggren et al. 264--146 X 2,361,369 10/1944Grebe et al. 264-290 X 3,217,361 11/1965 Ryan et al. 1814 FOREIGNPATENTS 3,715,483 9/1962 Japan.

ROBERT F. WHITE, Primary Examiner.

S. I. LANDSMAN, Assistant Examiner.

US. Cl. X.R.

